1. Field of the Invention
The present invention relates to an optical amplifier control apparatus that amplifies an optical signal using excitation light.
2. Description of the Related Art
In recent years, technologies such as OADM (Optical Add Drop Multiplexing) have been implemented for WDM (Wavelength Division Multiplexing) systems. OADM is becoming an essential technology especially for metro access networks that connect the user side and a backbone network. This means that by adding or dropping a desired wavelength to or from a wavelength multiplexed signal, the communication capacity can be changed depending on the user and thereby a network can be flexibly deployed.
FIG. 1 is an example of a drop configuration in this network. First, WDM signal light 9 is amplified by an optical amplifier in which erbium doped fiber (EDF) 3 is used for an amplifying medium. The amplifying medium is excited by laser diode (LD) 199 through optical coupler 59. Part of the WDM signal light that is input to EDF 3 is branched by optical coupler 69, photoelectrically converted by photo diode (PD) 78, and then input to control circuit 299. On the other hand, part of the WDM signal light that is output from EDF 3 is branched by optical coupler 69, photoelectrically converted by PD 79, and then input to control circuit 299. Control circuit 299 obtains an output control value of LD 199 corresponding to an optical amplification control scheme in use. For example, when a constant gain control scheme is implemented, control circuit 299 obtains an output control value for LD 199 such that input gain and output gain (the gains on the upstream and downstream sides of EDF 3) are constant.
The WDM signal light amplified by EDF 3 is demultiplexed into two parts by demultiplexer 99. Among them, necessary signal light is dropped as drop signal light 19. The other signal light passes through as through signal light 18.
FIG. 2 is an example of a drop configuration thought to be one candidate of the next generation drop configuration. In the configuration shown in FIG. 2, before WDM signal light 9 is amplified, it is demultiplexed into a plurality of paths by demultiplexer 799. FIG. 2 shows that WDM signal light 9 is demultiplexed into path 1 and path 2.
On path 1, optical coupler 66. PD 76, EDF 6, optical coupler 58, LD 198, optical coupler 67, PD 77, control circuit 298, and demultiplexer 98 are located. Their configuration is the same as that shown in FIG. 1. Demultiplexer 98 demultiplexes the WDM signal light amplified by EDF 6 into drop signal light 12 and through signal light 11.
On path 2, optical coupler 64, PD 74, EDF 7, optical coupler 57, LD 197, optical coupler 65. PD 75, control circuit 297, and demultiplexer 97 are located. Their structure is the same as that shown in FIG. 1. Demultiplexer 97 demultiplexes the WDM signal light amplified by EDF 7 into drop signal light 21 and through signal light 20.
In the configuration shown in FIG. 2, since a path can be selected together with the configuration shown in FIG. 1, a more flexible network than that of the configuration shown in FIG. 1 can be accomplished.
Examples of optical amplifiers that can be applied to the configurations shown in FIG. 1 and FIG. 2 are presented in Patent Literature 1 (JP2000-332330A, Publication) and Patent Literature 2 (JP2001-148669A, Publication).
An optical direct amplifier presented in Patent Literature 1 is composed of an optical amplification section, an excitation LD light source, a signal detector, and a control circuit. The optical direct amplifier presented in Patent Literature 1 operates in such a manner that when the number of channels of an input optically multiplexed signal is large, the number of excitation LD light sources is increased; when the number of signal channels decreases, the number of exciting LD light sources is decreased.
A wavelength multiplexed light repeater presented in Patent Literature 2 is composed of a plurality of optical transmission paths, excitation light sources, and EDFs. The wavelength multiplexed light repeater presented in Patent Literature 2 demultiplexes a WDM signal into signals with different wavelengths, optically amplifies them, and then multiplexes them.
Generally, optical amplifiers need to increase the output of an LD (light intensity of excitation light) corresponding to the number of input wavelengths of the WDM signal light. If the configuration shown in FIG. 2 is implemented, since it is likely that the entire WDM signal light is selected for path 1 or path 2, the LD on each path needs to output excitation light with light intensity that allows the entire WDM signal light to be amplified.
Thus, when the next generation drop configuration shown in FIG. 2 is implemented, since excitation light sources for optical amplifiers having the configurations presented in Patent Literatures 1 and 2 need to be located corresponding to the number of paths, a problem in which the cost rises in proportion to an increase of the number of paths arises.